As the number of homes and businesses that acquire computer equipment grows, the need for reliable, fast and cost-effective printers also continues to grow. In recent years, the print quality of inkjet printers has improved greatly to the extent that it now rivals that of laser printers.
The print head of an inkjet printer forms part of a print cartridge that is mounted in a carriage that moves the print cartridge back and forth across the paper. The print head includes many orifices, typically arranged in line aligned parallel to the direction in which the paper is moved through the printer and perpendicular to the direction of motion of the print head. Each orifice constitutes the outlet of a firing chamber in which is located a firing element such as a heating element or piezoelectric element. The firing element operates in response to an electrical signal to cause minute droplets of ink to be ejected from the orifice. Ink from a reservoir is supplied to the firing chambers through an ink manifold in the print head. When printing, a page is fed through the printer while the carriage moves the print cartridge, including the print head, back and forth across the page, to enable the print head to deliver ink to the paper. The rapid motion of the print cartridge during printing, variations in ink viscosity, temperature and altitude all contribute to difficulties in regulating the pressure at which the ink is delivered to each firing chamber in the print head.
In a so-called "on axis" arrangement, the print cartridge of an inkjet printer typically includes an ink reservoir located behind the print head. In newer inkjet printers that have an "off axis" arrangement, only a small amount of ink is stored in the print cartridge, and the main ink reservoir is positioned at a location remote from the print head. In the off-axis arrangement, an ink delivery tube delivers ink from the remote reservoir to the print cartridge. The ink delivery tube is considerably longer than the length of the ink delivery path in a typical on-axis print head. The off-axis arrangement reduces the mass of the print cartridge, which enables the print speed to be increased, or the power consumption of the carriage scanning mechanism to be reduced. The off-axis arrangement additionally provides the option of an increased ink storage capacity. However, the off-axis arrangement requires a pump or a gravity feed arrangement to deliver the ink through the ink delivery tube, and movement of the print cartridge causes the ink delivery tube to flex. Both of these factors compound the above-discussed difficulties in regulating the pressure at which ink is delivered to the print head.
In both on-axis and off-axis configurations, maintaining a predetermined ink pressure at the firing chamber is critical to the orifice delivering the expected amount of ink to the paper when the firing element operates. While some ink pressure regulation methods and devices regulate the ink pressure to a given absolute pressure, better ink pressure regulation methods and devices regulate the pressure of the ink in the firing chamber to a predetermined pressure below atmospheric pressure, since an ink pressure in the firing chamber equal to or greater than atmospheric pressure will cause ink to leak from the orifice. Therefore, the ink pressure in the firing chamber should be maintained at below atmospheric pressure to prevent the ink from leaking. On the other hand, too large a difference between the ink pressure in the firing chamber and atmospheric pressure will result in insufficient ink being delivered to the paper during printing. Accordingly, the difference between the ink pressure in the firing chamber and atmospheric pressure should be maintained within a predetermined range.
Spring-bag ink reservoir devices are widely used to regulate the ink pressure in the firing chambers of inkjet printers. In a spring-bag device, as the bag empties, the spring keeps ink stored in the bag at a constant pressure. This keeps the ink delivered to the firing chamber at a constant pressure. Additional measures are required to keep the difference between the ink pressure in the firing chamber and atmospheric pressure within a predetermined range.
U.S. Pat. No. 4,771,295 to Baker and assigned to the assignee of the present disclosure describes using a reticulated polyurethane foam placed in the ink reservoir to control ink pressure. Such an arrangement reduces the volume of ink that can be stored in a reservoir of a given size, however. Moreover, the regulated ink pressure varies due to variations in the size of the pores in the foam.
U.S. Pat. No. 4,794,409 to Cowger et al. and assigned to the assignee of the present disclosure describes an ink jet print head composed of a primary ink reservoir, a secondary reservoir and an orifice all interconnected by a porous member such as foam. As the pressure in the primary ink reservoir changes relative to ambient pressure, ink is drawn through the foam back and forth between the primary and secondary reservoirs.
U.S. Pat. No. 4,791,438 to Hanson et al. describes an inkjet print head having a primary ink reservoir, a secondary reservoir and a capillary member interconnecting the reservoirs. As the pressure in the primary ink reservoir changes relative to ambient pressure, ink is drawn through the capillary back and forth between the primary and secondary reservoirs.
U.S. Pat. No. 5,010,354 of Cowger et al., assigned to the assignee of the present disclosure, and entitled Ink Jet with Improved Volumetric Efficiency, describes a device in which a chamber containing a capillary volume element is directly coupled to an ink reservoir. Pressure in the ink reservoir is defined by a bubble generator coupled to the ink reservoir. A portion of the ink reservoir remote from the chamber is coupled to the print head. Pressure in the capillary volume element is greater than the normal sub-atmospheric pressure in the ink reservoir, but is less than atmospheric pressure. During operation at ambient pressures and temperatures within a normal range, ink does not enter the capillary volume element. When subject to temperatures and pressures outside the normal range, an increased pressure in the reservoir forces ink into the capillary volume element. This limits the pressure increase in the ink reservoir and prevents ink from being ejected from the print head. As the pressure in the ink reservoir returns to the normal range, the ink reservoir draws ink from the capillary volume element. In this arrangement, the capillary element acts a pressure surge protector for the ink reservoir. Since the capillary volume element controls the pressure in the ink reservoir, its volume must be of the same order as that of the ink reservoir. Thus, this arrangement suffers from the drawbacks that the mass of the capillary volume element increases the total mass of the print head, and the capillary volume element is complex to manufacture.
What is needed is an ink pressure regulator which can reliably maintain the pressure of the ink delivered to the firing chamber at a predetermined differential below atmospheric pressure or some other reference pressure. What is also needed is an ink pressure regulator that primarily provides such ink pressure regulation passively and that does not significantly increase the mass of the print head.